Self-renewal is the process by which at least one daughter cell retains the full developmental potential of its predecessor when stem cell division takes place. It is critically important for maintaining lifelong function of the blood forming (hematopoietic) system, as well as for increasing stem cell numbers after bone marrow transplantation. The Hox genes, a group of genes well known for their role in patterning the early embryo during development have recently been appreciated to also regulate self-renewal of the hematopoietic stem cell. The Hox genes comprise a large and highly related gene family, the result of duplication during mammalian evolution. Because there are up to eight gene copies within any Hox paralog group (2 alleles of 4 genes, HoxAn, Bn, Cn, and Dn), it is exceptionally difficult to generate the true null state, making the loss of function approach problematic. To date, gain of function studies have been done with retroviral transduction of Hox genes, however most Hox genes have untoward effects downstream of the HSC (leukemias and blockage to differentiation of particular lineages) if continually expressed. We have engineered into embryonic stem (ES) cells an inducible expression system that allows for regulated expression of Hox genes. We will characterize each Hox paralog group for effects on HSC self renewal during in vitro differentiation of these ES cell lines, and in so doing define the Hox codes for self-renewal of the HSC. Our long-term goal is a functional understanding of the Hox code for self-renewal, encompassing an understanding at the molecular level of what makes the different paralog groups different, the identification of key downstream target genes, and ultimately an understanding of how these transcriptional responses translate into phenotypic effects. This work will provide a rational basis for the manipulation of hematopoietic stem cells in the transplantation context, in particular facilitating the use of embryonic stem cells as a source of cellular material for transplantation.